Exposure question.

4labs · August 23, 2005

I was experimenting this morning and I decided to take one exposure for each apeture of my lens. On the wider apetures I had no trouble but I noticed as I stopped down on the really smaller apetures my metering got all screwy and I started to blow highlights. I am sure there is an explanation for what is happening. I was under the asumption that if I was on apeture priority as I went from F2.8 to F22 the exposure would be picked by the camera automatically and stay relatively constant.

This is the scene at F11. I had to underexpose a stop and as I went to F22 I had to underexpose by 2 stops to keep from blowing everything out? What am I missing?.

wxwax · August 23, 2005

A neophyte guess would be that the camera's brain over-compensated as you stopped down and let in less light.

4labs · August 23, 2005

wxwax wrote:

A neophyte guess would be that the camera's brain over-compensated as you stopped down and let in less light.

Thnxs Sid I guess what I wanted to know was is this expected?

wxwax · August 23, 2005

4labs wrote:

Thnxs Sid I guess what I wanted to know was is this expected?

I should think not.

I wonder if you'd get the same result if there weren't the sun glaring into the sensor?

bfjr · August 23, 2005

1st great shot love the depth and color here

2nd what metering pattern, think that might play a big role

3rd how's by you ? Everything AOK I hope ??

Harryb · August 23, 2005

Another possibility is that the light changed in between the two exposures. The light changes rapidly at sunrise and sunset. esp. sunrise which was what you were shooting. I have had to change my ev setting from shot to shot on more than one occasion.

4labs · August 23, 2005

Harryb wrote:

Another possibility is that the light changed in between the two exposures. The light changes rapidly at sunrise and sunset. esp. sunrise which was what you were shooting. I have had to change my ev setting from shot to shot on more than one occasion.

Thnxs Harry, I am not sure if that is the problem but I will definately try and find something to read up on it. Two whole stops in a minutes time just doesn't seem right but it might be.. (Maybe Andy or Shay can chime in ) Hint Hint..

Safari · August 23, 2005

4labs wrote:

Two whole stops in a minutes time just doesn't seem right but it might be.

Not Andy or Shay, but for what it's worth -
(a) I think the light could easily change by a couple of stops at that time of day. Earlier or later, probably not.
(b) What ELSE was changing? If you were getting more exposure at smaller apertures, did the shutter speed reduce dramatically or did the ISO go up?

pathfinder · August 23, 2005

The EV difference between just before sunrise, during sunrise, and after the sun has cleared the horizon is a lot more than just 2 fstops - I'll bet it is closer to 4 - 6 or more. When the sun is splitting the horizon like your shot, the exposure will be changing second by second.

If you were shooting in Av mode and ONLY changed the aperature, the camera would adjust the shutter speed to correlate regardless of how the light changed though as it is metering real time. But the EV values will change dramatically as the sun rises.

4labs · August 23, 2005

Hey Safari and Pathfinder thnxs so much for taking the time. I think what surprised me is I thought the exposure woudl be similiar to taking pictures in teh snow and I woudl haev to overexpose and not under expose to get it right.

4labs · August 23, 2005

Safari wrote:

Not Andy or Shay, but for what it's worth -
(a) I think the light could easily change by a couple of stops at that time of day. Earlier or later, probably not.
(b) What ELSE was changing? If you were getting more exposure at smaller apertures, did the shutter speed reduce dramatically or did the ISO go up?

The shutterspeed reduced darmatically, much more than I thought. I quickly was able to compensate but I was just surprised by how much.

Safari · August 24, 2005

4labs wrote:

The shutterspeed reduced darmatically, much more than I thought. I quickly was able to compensate but I was just surprised by how much.

2 things may help understand why there such a **BIG** change.
(1) The standard f-stops are an exponential index, not linear. Each stop indicates twice the exposure of the previous (or 1/2 in the other direction). Note, though, that some lenses add intermediate stops between the standards. Each full-stop aperture increase corresponds to reducing the shutter speed by HALF.
(2) At sunrise over a period of just a few minuntes you are transitioning from very low light (**NO** sun) to **LOTS** of light - and the opposite at sunset. Clouds will extend this transition period by reflecting light when the sun is below the horizon, but without clouds the change can occur very quickly.
Imagine lighting one candle in a small dark room, then lighting a second, and suppose that's the equivalent of 1 f-stop and 1 minute at sunrise. 1 minute later, 2 more candles or another f-stop. In very few minutes you've got a big bunch of candles and f16, but it will take lots more candles now to reduce your aperture one more f-stop to f22. Human eyes and brains respond much less mechanically than cameras so you must be very cautious about trying to relate perceived lighting changes to exposure readings on a camera.

4labs · August 24, 2005

Safari wrote:

2 things may help understand why there such a **BIG** change.
(1) The standard f-stops are an exponential index, not linear. Each stop indicates twice the exposure of the previous (or 1/2 in the other direction). Note, though, that some lenses add intermediate stops between the standards. Each full-stop aperture increase corresponds to reducing the shutter speed by HALF.
(2) At sunrise over a period of just a few minuntes you are transitioning from very low light (**NO** sun) to **LOTS** of light - and the opposite at sunset. Clouds will extend this transition period by reflecting light when the sun is below the horizon, but without clouds the change can occur very quickly.
Imagine lighting one candle in a small dark room, then lighting a second, and suppose that's the equivalent of 1 f-stop and 1 minute at sunrise. 1 minute later, 2 more candles or another f-stop. In very few minutes you've got a big bunch of candles and f16, but it will take lots more candles now to reduce your aperture one more f-stop to f22. Human eyes and brains respond much less mechanically than cameras so you must be very cautious about trying to relate perceived lighting changes to exposure readings on a camera.

Thank you for your very thoughtful explanation.

Safari · August 24, 2005

You're very welcome.....
..... glad it was some help.

Jekyll & Hyde · August 26, 2005

4labs wrote:

I was under the asumption that if I was on apeture priority as I went from F2.8 to F22 the exposure would be picked by the camera automatically and stay relatively constant.

J: Hi 4labs (hey, my brother has 4 labs too).

H: I think your initial gut feeling was correct here. Not that the others haven't made good points, but I don't think they apply to what you observed.

J: The camera should, and does, choose the correct shutter speed to keep the exposure constant as you change the aperture in Aperture Priority mode. If it didn't, it would deserve to be tossed in the bay.

H: You also didn't run out of available shutter speeds or ISO here. In any case, that would have resulted in an Underexposed image.

J: I think that bfjr is on the right track here. Though I can't tell for sure without the EXIF.

bfjr wrote:

1st great shot love the depth and color here
2nd what metering pattern, think that might play a big role

H: 1st. Yes beautiful shot.

J: 2nd. Matrix metering can play havoc with your exposure. Moving the camera a couple of degrees can change the exposure by several stops (as a "backlight" algorythm (or some other) kicks in).

H: Spot metering can have the same effect.

J: Good old Center Weighted metering is about the most stable and predictable, though it too can be influenced by small changes in framing (especially if there is a specular light source in the composition).

H: The rising sun does change the E/V very quickly and significantly to be sure. However in Aperture Priority mode the camera will compensate, as you have already noted. It seeks out that 18% grey with an uncanny single-mindedness.

J: Now the camera's metering can in fact change the exposure in some parts of the image quite drastically in this situation (even if your camera was on a tripod and your framing didn't change), but it's because the increasing brightness of the sun is changing the nature of the scene before you. From a low contrast image, to a high contrast image. Bingo, back light compensation et al. This will be the most pronounced if you are in Matrix Metering mode.

H: Sorry if this is kinda confusing (and not explained well). The bottom line is that small changes in framing can cause large changes in exposure, and also that subtle changes in light can cause large changes in exposure. One, or the other, or both were at play here.

J: Do another test. Put your camera on a tripod (to keep the framing the same), and shoot the images in quick succession to keep the light levels constant.

H: I think you'll find your answers. Or else that camera wil go a-swimmin'.

Happy snappin'
J&H

Safari · August 26, 2005

4labs wrote:

what surprised me is I thought the exposure woudl be similiar to taking pictures in the snow and I woudl have to overexpose and not under expose to get it right.

Well, the concept is the same. In both situations you have a high-contrast image. The question: which part of the image is important? In snow scenes you generally want to capture the darker parts of the image, you don't care if the highlights (snow) are blown, so you must overexpose because otherwise the camera will meter largely on the snow. But in this lovely picture it's the highlights (the rising sun and reflections) that are important and you are willing to sacrifice some detail in the shadows - so you must underexpose to avoid blowing the highlights.

Jekyll & Hyde · August 26, 2005

Safari wrote:

Well, the concept is the same. In both situations you have a high-contrast image. The question: which part of the image is important? In snow scenes you generally want to capture the darker parts of the image, you don't care if the highlights (snow) are blown, so you must overexpose because otherwise the camera will meter largely on the snow. But in this lovely picture it's the highlights (the rising sun and reflections) that are important and you are willing to sacrifice some detail in the shadows - so you must underexpose to avoid blowing the highlights.

J: Your concept is sound, but I was responding to the original question 4labs had regarding why the camera was not maintaining the same exposure as he adjusted the aperture (in Aperture Priority mode).

4labs wrote:

I was under the asumption that if I was on apeture priority as I went from F2.8 to F22 the exposure would be picked by the camera automatically and stay relatively constant.

H: The exposure should stay exactly constant. I put forth some factors that would account for the "mis-metering" that he experienced.

J: From an artistic standpoint, your info is just super. And I think 4labs has the concept down pat.

J&H

Exposure question.

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